Percutaneous transluminal coronary angioplasty (PTCA) procedures are becoming well known in the medical field as an efficacious way to clear a stenosis from a blood vessel. In general, PTCA procedures require the application of pressure against the stenotic segment of a vessel or artery until the stenosis is sufficiently compromised to permit improved fluid flow through the vessel. Typically, PTCA is accomplished using a balloon catheter to dilate the stenotic segment of the vessel. The result is that the obstructive tissue causing the stenosis is either stretched, compacted or cracked to clear the obstruction.
Not surprisingly, several devices have been proposed which are useful for PTCA procedures. Typical of such devices is the dilatation catheter disclosed in U.S. Pat. No. 4,706,670 to Anderses et al. Although these devices can be effective for clearing obstructive tissue from a vessel, their use requires a great deal of intuitive skill to obtain the desired result while avoiding or minimizing damage to the vessel in the process.
Unfortunately, the application of excessive pressure in the stenotic segment can cause unnecessary damage to the vessel. Further, it is believed that a vessel which has been excessively stretched is more likely to restenose than one which has not been subjected to such stretching. Nevertheless, sufficient force must be applied to compromise the stenosis. The amount of force required to compromise a stenosis is, however, not easily determined and will vary from patient to patient. Also, the amount of force required will vary according to the characteristics of the obstructive tissue being compromised. Furthermore, since PTCA procedures typically require successive uses of increasingly larger balloons with consequent additional trauma to the patient, it is desirable to know when the PTCA procedure has either been effective or is likely to start seriously damaging the vessel. Thus, proper control of the PTCA procedure is essential.
One reported PTCA procedure, employing a balloon catheter, uses comparative readings of fluid pressure and fluid volume in the balloon to determine the state of the stenotic segment being compromised. This procedure, as set forth in the article, "In vivo Assessment of Vascular Dilatation During Percutaneous Transluminal Coronary Angioplasty" by Jain et al., Vol. 60, The American Journal of Cardiology, November 1987, suggests that the pressure-volume readings of a balloon catheter in response to the resistance of a stenosis is predictable according to the characteristics of the stenosis and the consequent mechanism by which the stenosis is compromised. Specifically, this procedure is used to observe and identify the mechanical behavior of the dilatation process in comparison with an expected response. U.S. Pat. No. 4,651,738 to Demer et al. for an invention entitled "Method and Device for Performing Transluminal Angioplasty" also discusses the use of this same pressure-volume relationship to observe PTCA dilatation mechanisms.
The present invention recognizes that, though pressure-volume observation can tell how the stenosis is reacting to PTCA, pressure-volume information alone does not provide a complete picture of the actual situation. Specifically, pressure-volume information overlooks vessel compliance as a function of pressure and time (i.e. resilience of the vessel). It happens in PTCA procedures, however, that information concerning vessel compliance can be crucial for determining when stenosis clearance has occurred and, subsequently, whether to continue dilatation. For example, if the vessel is at or near its elastic limit (i.e. there is little, if any, vessel compliance) then the PTCA procedure should be stopped because further stretching of the vessel will cause damage and be of minimal benefit. On the other hand, even though it may appear that dilatation has been successful, if there is still vessel compliance, it may be advantageous to continue stretching the vessel to more effectively open the stenosis.
The present invention recognizes that PTCA procedures are optimally effective when the full range of vessel compliance is utilized. Further, the present invention recognizes that by monitoring the pressure drop and the time rate of change of fluid pressure in a balloon catheter, rather than relying solely on pressure-volume information, the compliance of the vessel segment surrounding the balloon can be determined. Further still, the present invention recognizes that PTCA can be efficaciously accomplished using only information pertaining to the pressure drop and the rate of change of fluid pressure with time (.DELTA.p/.DELTA.t) in a balloon catheter. The present invention also recognizes that .DELTA.p/.DELTA.t information is useful in a variety of medical procedures which require vessel dilation.
Although the above discussion has focused on PTCA procedures, it is to be appreciated that other medical procedures may also benefit from the present invention. Indeed, the present invention recognizes that whenever a body vessel is to be dilated by a balloon catheter, information pertaining to the rate of change of fluid pressure with time (.DELTA.p/.DELTA.t) in the balloon can be useful. This is so regardless of the vessel involved.
In light of the above, it is an object of the present invention to provide an apparatus for use in vessel dilatation procedures (including PTCA) which displays the change of fluid pressure in a balloon catheter over a selected period of time to indicate vessel compliance. Another object of the present invention is to provide an apparatus which can effectively indicate whether further vessel dilatation during a medical procedure should be avoided. Yet another object of the present invention is to provide an apparatus for use in vessel dilatation procedures which chronicles fluid pressure variations in a balloon catheter during the procedures. Still another object of the present invention is to disclose a method for using the time rate of change of fluid pressure in a balloon catheter to determine vessel compliance during vessel dilatation procedures. Another object of the present invention is to provide an apparatus for performing vessel dilatation procedures that is simple to use, relatively easy to manufacture and cost effective for its intended purposes.